1. Field of the Invention
This invention relates generally to the recording and/or reproducing systems and more specifically to an improved system for recording and/or reproducing digital data.
2. Description of the Prior Art
In our complex society, it is highly desirable to be able to accumulate, process and transmit various types of information or data. This data can be used, for example, to maintain inventories, sales records, or to provide other types of accounting information. It has been found that the accumulation, processing, and transmitting of this data can be facilitated by expressing the data in a digital format. Thus the data can be expressed as a plurality of digital words containing digital "ones" and digital "zeros" having positions within the digital word corresponding to the information to be transmitted. For example, in a common binary representation, the numeral 13 is expressed in digital "ones" and "zeros" as the digital word 1101.
Various types of digital processing apparatus, such as computers and associated hardware, have been developed to process the digital information. These apparatus typically include a storage device which permits storage of the digital data for an extended period of time, and retrieval of the digital data at some time in the future. Many of these storage devices include tapes, disks, drums, etc. which have a material coated on their surfaces which can be magnetized to provide flux changes from a first direction to a second direction and from the second direction to the first direction in accordance with the digital information.
Of course it is desirable to maximize the amount of information that can be stored by a given storage device. A primary limitation on this storage capacity is the resolution of the storage device which is based upon the number of flux changes that can be distinguished along a given length of the magnetized surface. The closer the flux changes are to each other, the less distinguishable they become. Thus, the resolution of the device is dependent upon the packing density of the data.
Consider for example a system including a storage device which provides a cell on the magnetizable surface for each of the bits in the digital data. Each cell is provided with a flow of flux in the first or second direction depending upon the binary characteristic, zero or one, of the associated data bit. In order to accommodate the same type of binary characteristic for adjacent bits of the digital signal, the flux has been returned to a zero or neutral state at the end of each of the data cells. This "return to zero" method of encoding has been undesirable since two flux reversals have been associated with each cell of the storage device. For this reason, the amount of surface area devoted to each bit of the data has been significant in order to distinguish adjacent flux changes.
As used herein, a cell is defined as that portion of the magnetic surface of the storage device which would be devoted to the storage of a single bit of the digital data if the storage device were magnetized in accordance with the "return to zero" method of encoding.
To maximize the separation between adjacent flux changes and therefore reduce the surface area devoted to storage of each bit of the data, the prior art includes various other systems for encoding the digital data.
In a "non-return to zero" method of encoding, the flux has been oriented in one of the first or second directions and a flux change has been produced only when the binary characteristics of the digital data have changed. For example, with the flux oriented in the first direction, a change from a digital "zero" to a digital "one" might be represented by a reversal of the flux to the second direction. A change from a digital "one" to a digital "zero" would be represented by a flux change to the first direction.
A modification of the "non-return to zero" system has provided a flux change for each of the digital "ones" in the data. Thus if the flux is initially oriented in the first direction, the next digital "one" in the data will reverse the flux to an orientation in the second direction. Similarly, the next digital "one" in the data will result in a reversal of the flux back to the first direction. These "non-return to zero" systems for encoding the data have been somewhat effective in increasing the packing density by providing not more than one flux change per cell.
Although it is desirable to minimize the number of flux changes per cell, it is also desirable to provide some means for synchronizing the retrieval of data from the storage device with the other operations in the system. With the "return to zero" system for encoding, the flux changes associated with each of these cells could be used for synchronizing. In the "non-return to zero" systems, however, there may be no flux changes for an extended period of time if the binary characteristics do not change. To provide synchronization for these "non-return to zero" encoding systems, a separate clock channel has been provided on the magnetic surface of the storage device. A separate clock channel is undesirable, of course, since it uses space on the surface of the storage device which could otherwise be devoted to the storage of data.
In a phase mode system for encoding the data, a digital "one" is recorded as a single cycle of a first square wave and a digital "zero" is recorded as a single cycle of a second square wave shifted 180.degree. from the first square wave. Thus a flux reversal in one direction indicates the digital "one" and a flux reversal in the opposite direction indicates a digital "zero". If follows that a flux reversal in one of the first and second directions is provided for each cell of the digital data. In such a system, the flux reversals associated with the data can be used to synchronize a clock so that a separate clock channel need not be provided. One disadvantage of this system is that two flux reversals are often necessary to record a single bit of the data. As noted, this limits the maximum storage density of the device. Furthermore, it is necessary to sense the direction of the flux reversals in order to distinguish the digital "ones" and "zeros".
A modified phase mode system of encoding is described in U.S. Pat. No. 3,108,261 issued to Miller on Oct. 22, 1963. In this system, the data cells are divided into two flux change positions, one at the center of the cell and one at the end of the cell. A change of flux at the end of the cell corresponds to the digital "1" while a change of flux at the center of the cell corresponds to the digital "0". In this system, the first digital "0" appearing after a digital "1" is ignored. As opposed to the phase mode system, this modification provides flux changes which may occur no further apart than one cell. Though this system provides a packing density as great as any of the systems in the prior art and additionally provides self-clocking, an increase in the packing density is always desirable.